class Solution2 {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        //出口：哪个指针为空，返回另一个指针
        if(list1==null)
            return list2;
        if(list2==null)
            return list1;
        //找头
        if(list1.val<list2.val) {
            //list1为头
            list1.next = mergeTwoLists(list1.next,list2);
            return list1;//返回值
        } else {
            //list2为头
            list2.next = mergeTwoLists(list1,list2.next);
            return list2;
        }
    }
}
